LEAF SURFACE WETNESS AND GAS-EXCHANGE IN THE POND LILY NUPHAR POLYSEPALUM (NYMPHAEACEAE)

As part of a continuing study of the effects of leaf surface wetness o n gas exchange, the occurrence of leaf surface wetting by dewfall and associated effects on photosynthesis were evaluated for floating and a erial leaves of the pond lily Nuphar polysepalum Engelm. Because of ni ghttime radiation exchange with a cold sky, high humidity, and the pre sence of adaxial stomata, we predicted that pond lily leaves would be particularly susceptible to wetting events such as dewfall. A substant ial reduction in net photosynthesis (up to 20%) occurred for leaves th at were experimentally misted to simulate leaf wetting by dewfall. Aer ial leaves remained below dewpoint temperatures for long periods on cl ear nights. However, floating leaves rarely approached dewpoint temper atures at night because minimum nighttime temperatures of leaves were up to 10 C warmer than air temperature. Thus, floating leaves of N. po lysepalum did not experience dew formation primarily because of strong thermal coupling to a substrate (water) that was much warmer than air temperature at night. This coupling to a warmer substrate prevented a potentially strong inhibition of photosynthetic CO, exchange the foll owing morning.